Calc Q value

[planit]

I need to calculate the amount of cfs overflowing a road
at a maximum of twelve inches. How can I do this? For
example... I have 12" of water running over a road sloped a 2% and 26' wide, how do I calc the CFS for it.

 

[Bayou]

the easiest way is to use the weir equation:
Q = CL(H^1.5)

L = weir length
H = head above weir
C = weir coefficient (for roads, typical is 2.65)

 

I would calc 26*2.5 getting 68.9. Then calc 12 to the power of 1.5, which comes out to 41.57, and the multiply the two, getting 2864 CFS overflowing the road...it seems high, am I doing this right?

 

[Bayou]

To the last post--please keep your units the same.
Q = 2.65*26*(1^1.5) = 69 cfs

 

[Briansch]

What I gathered from the description was the flow was going perpendicular to a road that was 26' wide. This would make the weir length 26'. The weir crest would then be the road length, I think. Looking quickly at my open channels book, for a LONG weir(L/H>3), the flow should be based on brink depth or the depth of water just ahead of the "weir" edge by q = 1.65 * yb *(g*yb)^0.5 which is equal to 9.36 * yb^1.5. If we assume yb is 12" then q = 9.36 cfs per foot of length of roadway. Would you agree? The above equation would not be applicable if the flow was submerged.

Briansch

 

[planit]

Thank you Briansch...that's exactly what I needed.

 

I have a similar problem. I have a a road cross section
like this:

A negative slope of 6.81% down to a tangent 20' vertical curve connecting to a tangent flat road lower than the BVC. The road is 40' wide and then another tangent vertical curve 20' wide upward to a tangent slope of 4%.
At a 100 year event, the maximum amount of water overflow needs to be 12". I need to calc how much CFS the road will carry at 12" overflow.

Example:

slope in (negative) slope up (+)

\ /
\ Water Overflow /
\------------------------/
\ Bottom of Road /
---------------------

 

How would I calc the above problem?

 

[icejammer]

Actually, if I understand the question, weir flow wouldn't apply to this situation. For situations where a broad-creasted weir exists, if the flow depth divided by the length of the broad-crest (measured in the direction of flow) is less than 0.08, then flow over the weir crest is subcritical and weir flow cannot be used. In this case you have 1 ft/26 ft = 0.038. The depth of flow is so shallow relative to the length of the spillway that energy losses cannot be neglected. In other words, your depth of flow will not be a constant depth across the roadway unless the flow is accelerating.

I'm afraid you'll need to run a backwater profile to determine your discharge and change your discharge until you get the depth of flow you are looking for.